6-2.2 EXCLUSIONS TO SINE SWEEP TOLERANCES

Three types of exceptions to the general sine tolerance specified in
Chapter 5, Table 5.1 , have
been identified and will be discussed in detail in subsequent paragraphs:

a. Compensation Factor Exclusions

b. Bandwidth Exclusions

c. Lower Tolerance Exclusions

6-2.2.1 Compensation Factor Exclusions. As explained in detail in Chapter 4,
para. 4-2.4 ,
compensation factors have been applied to the random tape to correct tape deck
characteristics and shifts in resonant frequencies since these factors have
not been included in the sine portion of the tape, these deviations should be
considered to predict their effect when running the random noise test. For
example, Chapter 4,
Figure 4-3 , shows a total for linearity and variance compensation factors of
- 4.1 dB for filter No 13 (213 Hz). Therefore a +7 dB spike at this
frequency in the sine sweep will be reduced to a + 2 9 dB spike in the random
portion of the test, which is within acceptable tolerances.

6-2.2.2 Bandwidth Exclusions. As discussed in Chapter 1, Theory of Operation, the measurement
of a random spectrum is accomplished by averaging the vibration energy
over a finite band of frequencies with maximum bandwidths ranging from 25
to 100 Hz. For example a + 7 dB spike of 30 Hz duration at 1500 Hz in the sine
sweep would increase the random spectrum in a 100 Hz bandwidth (1450 to 1550
Hz) by only +2 dB which would be within the required random spectrum tolerances.
A tabulation of sine-random equivalents for different bandwidths when
measured at the half signal level (- 6 dB) is presented in Table 6-1 and
6-2. The deviations noted during the sine sweep (after correction for
compensation factors - para. 6-2.2), should be reduced by the bandwidth factor
tabulated in Table 6-1 and 6-2 (but not to less than 0 dB, the nominal noise
level).

FrequencyRange

MaximumBandwidth

dB Change in Random Deviation Per Sine BW

10

20

30

40

50

60

75

100

150

20 to 200 Hz

200 to 1000 Hz

1000 to 2000 Hz

25 Hz

50 Hz

100 Hz

- 4

- 7

- 10

- 1

- 7

- 10

0

- 2

- 5

0

- 1

- 4

0

0

- 3

0

0

- 2

0

0

- 1

0

0

0

0

0

0

Table 6-1. Sine-Random Equivalents for Analysis
Bandwidths (Maxima)

Resonant BW/Filter BW

10

20

40

60

75

25

50

100

25

50

100

25

50

100

25

50

100

25

50

100

- 1 dB

- 2 dB

- 3
dB

- 4 dB

- 5 dB

- 10 dB

- 15 dB

- .37

- .69

-
.97

- 1.20

- 1.39

-
1.94

- 2.13||-
2.22

- .18

- .33

-
.46

- .56

- .64

- .84

- .93||-
.97

- .09

- .16

-
.22

- .27

- .31

- .41

- .44||-
.46

- .78

- 1.52

-
2.21

- 2.85

- 3.44

- 5.52

-
6.47||-
6.98

- .37

- .69

-
.97

- 1.20

- 1.39

- 1.94

- 2.13||-
2.22

- .18

- .33

-
.47

- .56

- .64

- .86

- .93||-
.97

-

-

-

-

-

-

-

-

- .78

- 1.52

-
2.21

- 2.85

- 3.44

- 5.52

-
6.47||-
6.98

- .37

- .69

-
.97

- 1.20

- 1.39

- 1.94

- 2.13||-
2.22

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

- .57

- 1.09

-
1.54

- 1.95

- 2.29

- 3.49

-
3.78||-
3.98

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

- .73

- 1.41

-
2.03

- 2.61

- 3.12

- 4.88

-
5.62||-
6.02

Example:At a frequency of 1250 Hz, a
spike of -5 dB (minia value) of 40 Hz bandwidth measured at the half
signal level (+ 6 dB), will result in an equivalent random - 1.39 dB spike
when analyzed with a 100 Hz bandwidth filter.

Table
6-2. Sine-Random Equivalents for Analysis Bandwidths (Minima)

6-2.2.3 Lower Tolerance Exclusions. Since reductions in amplitude, particularly at higher
frequencies, do not threaten the structural integrity of the test article or
the validity of the random test, we can apply less stringent tolerances in this region. Since most significant structural resonance occur below 1000 Hz, the allowable amplitude drop outs above 1000 Hz will be - 12 dB with a maximum cumulative bandwidth of 300 Hz.